The present invention relates to a device, as well as a process, for determining the position of the border areas between different mediums, specifically for determining the layer thickness of the uppermost of two superimposed filling materials inside a container by means of electromagnetic waves.
Devices and processes of this kind are known from WO 00/43739 and WO 00/43806, among other sources. To be sure, these documents describe only a functional dependence of the dielectric constants xcex5r on the reflection factor, and thus the relation of the voltage that returns on the cable to the departing voltage. Furthermore, neither of these documents discloses attenuation losses in the line.
The present invention is therefore based on the problem of elaborating the processes and devices named in the two documents, in such a way that the border areas between the two mediums can be more precisely determined.
First, however, the technological background of the present invention will be elucidated.
For some time the measurement of filling-levels in industry has employed measuring systems which precisely determine the distance between a sensor and the filling material, as based on the measured transit time of electromagnetic waves that travel from a sensor mounted on the container lid above the filling material to the surface of the filling material and back again. Thus, given the container height, conclusions can be reached about the filling level in the container. Sensors of this kind, which are known as filling-level radar sensors, are based overall on the property exhibited by electromagnetic waves of propagating at constant speed within a homogeneous, non-conductive medium and of being at least partially reflected at the border area between different mediums.
Different radar principles are known to the prior art for determining the wave transit time. The two principally applied methods are pulse radar and FMCW radar. Pulse radar makes use of the pulsed amplitude modulation of the emitted wave and determines the direct period of time between transmission and reception of the pulses. FMCW radar determines the transit time indirectly by transmitting a frequency-modulated signal and ascertaining the difference between the transmitted and the received momentary frequency.
In addition to the different radar methods, various frequency ranges can be used for the electromagnetic waves, depending on the application. For example, there are pulse radar systems with carrier frequencies in the range between 5 and 30 GHz, as well as those that operate in the base band as so-called monopulse radar systems without a carrier frequency.
Furthermore, a series of processes and devices are known which conduct the electromagnetic wave to the surface of the filling material and back again. A basic distinction is made here between a wave emitted into space and one conducted through a cable. Examples of the first type have an antenna that emits the wave with a sufficient degree of focus in the direction of the filling material and then receives it back again. This kind of sensor system is described, e.g., in DE 42 40 492 C2. Radar sensors which guide the electromagnetic wave through a cable to the reflection point and back again are often referred to as TDR (time domain reflectometry) sensors. The cable employed here can have any form customary in high frequency technology. By way of example, single-wire cables, as described in DE 44 04 745, may be mentioned, as well as waveguides, as described in DE 44 19 462.
In addition to the conventional filling-level radar measurements, which determine only the position of the border area between the filling material and the gaseous space above it (air in the usual containers), there are applications in which the goal is determine the position of the bordering layer between two different filling materials, or the layer thickness of the upper layer. Since every border layer between two mediums with different dielectric constants produces an echo, a radar sensor in this case will receive reflections from several points. In addition to the usual reflection at the border area between the gas and the uppermost filling material, an echo will arise at the border between the two filling materials. Under certain circumstances, other echoes may follow from other border areas of filling materials and also from (metallic) container floors. With the appropriate signal evaluation it is possible in any case to clearly identify the echo that results from the reflection at the gas/uppermost filling material border and the one that results from the reflection at the border leading to the next filling material. The sought after layer thickness of the uppermost filling material can be determined from the interval of time that separates the two echoes if the propagation speed of the wave-within this filling material is known. This propagation speed v depends on the dielectric constants xcex5r of the filling material and the permeability xcexcr of the filling material. The following formula applies in a calculation based on the propagation speed V0 in a vaccum.                     v        =                              v            0                    ·                                    1                                                ϵ                  r                                ·                                  μ                  r                                                                                        (                  equation          ⁢                      xe2x80x83                    ⁢          1                )            
Since the filling materials almost never have a magnetic property the permeability is known (xcexcr=1), and the dielectric constant remains the only unknown. In the past it has often been very difficult to determine this constant, since the user of filling-level sensors frequently has no knowledge of the material properties of the filling material. In addition, many containers are alternately filled with materials whose dielectric constants differ, and this necessitates continuous correction through renewed input of the value. Heretofore a further problem has resulted from the fact that the dielectric constant of many mediums is both temperature-dependent and also dependent on the frequency of the electromagnetic wave. Thus even if this material constant is known for a given temperature and a defined frequency range, for example several kilohertz, it can be assumed that for other filling material temperatures and sensor frequencies in the high and maximum frequency range the measuring result based on this predetermined value for the constant will not provide an exact outcome.
With the present invention it is possible to avoid the manual input of the dielectric constants that has heretofore been necessary in measuring separating layers with electromagnetic waves. Instead, a process is proposed, along with a device corresponding to this process, which makes it possible to determine the actual parameters that are dependent on the filling material and that are needed for ascertaining the layer thickness.
Furthermore this invention can be applied when a radar sensor works according to a process like that described in DE 42 33 324. Instead of directly determining the position of a filling level surface from a reflection that under certain circumstances may be relatively weak for filling materials with a low dielectric constant, the echo from the container floor is located, which is usually strong in this case. With a knowledge of the dielectric constants and the distance to the floor for an empty container it is easy to ascertain the filling height of the container.
Whereas the distance to the floor can be measured by a sensor without difficulty in the case of an empty container, or can be input a single time, the description just given applies to the dielectric constant. For radar determination of the filling level according to this method of floor tracking, the present invention also allows input of the dielectric constant to be advantageously replaced by a internal determination of the needed computing factor using a sensor.
With the present invention it is thus possible to determine the dielectric constant from the reflection factor at the border area of the filling material whose dielectric constant is being sought. The reflection factor, in turn, can be determined by measuring the echo amplitude, while incorporating a knowledge of the wave propagation of the electromagnetic wave. The decisive parameter of wave propagation here is the wave resistance ZL. It is defined in general as the location-independent quotient of voltage and current at each point in a cable, and can be calculated as                               Z          L                =                              k            ·                                                                                μ                    0                                                        ϵ                    0                                                  ·                                              ⁢                                                    μ                r                                            ϵ                r                                                                        (                  equation          ⁢                      xe2x80x83                    ⁢          2                )            
where k is a constant dependent on the wave propagation, and xcexc0 and xcex50 are the magnetic and electric field constant. The permeability xcexcr and the dielectric constant xcex5r designate the material constants of the field-conducting medium. The factor xcexc0/xcex50 designates the free field impedance Z0 of the propagation of a wave in a vacuum and equals 377 xcexa9.
In the further investigations it is assumed that the wave-conducting medium has no magnetic component and therefore that xcexcr=1.
The constant k is dependent on the type of wave propagation, i.e., on, for example, the form in which an electromagnetic wave is conducted. For a radar sensor with a coaxial measuring probe the wave resistance equals                               Z          L                :=                                            Z              0                        π                    ·                      1                                          ϵ                r                                              ·                      ln            ⁡                          (                                                D                  A                                                  d                  l                                            )                                                          (                  equation          ⁢                      xe2x80x83                    ⁢          3                )            
where
DA=diameter of the outer conductor
dI=diameter of the inner conductor
Other examples for wave resistances of conventional measuring probes:
Two-wire Probe:                               Z          L                =                                                            Z                0                            π                        ·                          1                                                ϵ                  r                                                      ·            arc                    ⁢                      xe2x80x83                    ⁢          osh          ⁢                      xe2x80x83                    ⁢                      (                          s              d                        )                                              (                  equation          ⁢                      xe2x80x83                    ⁢          4                )            
where
s=spacing of conductor
d=dimeter of conductor.
Waveguide with wave in fundamental mode:                               Z          L                =                              c                                          1                -                                                      (                                                                  λ                        0                                                                    λ                        c                                                              )                                    2                                                              ·                      1                                          ϵ                r                                                                        (                  equation          ⁢                      xe2x80x83                    ⁢          5                )            
where
c=constant
xcex0=wavelength in air
xcexc=boundary wavelength of air-filled waveguide
Finally, for wave propagation in free space the wave resistance is                               Z          L                =                              Z            0                    ·                      1                                          ϵ                r                                                                        (                  equation          ⁢                      xe2x80x83                    ⁢          6                )            
Thus given known forms of the wave propagation and conductor design, there is a clear relationship between the wave resistance ZL and the dielectric constant xcex5r of the propagation medium. In general in can be assumed that the wave resistance is inversely proportional to the square root of xcex5r.
If the propagation medium is a gas, a good approximation will result if the dielectric constant of the gas to be put at 1:
Zgas=ZL(xcex5r=1)xe2x80x83xe2x80x83(equation 7) 
When the propagation medium is a filling material with dielectric constant xcex5r, its wave resistance can be expressed as:                               Z                      L            ,                          filling              ⁢                              xe2x80x83                            ⁢              mterial                                      =                              1                                          ϵ                r                                              ·                      Z            Gas            -                                              (                  equation          ⁢                      xe2x80x83                    ⁢          8                )            
The reflection factor of the wave at the border area is defined as the ratio of the amplitudes of the electrical field strengths of the returning wave to the departing wave at the point of reflection. Since an amplitude ratio is involved, the amplitude of the electric field strength of the wave will be replaced by a proportionate measure of voltage U in the following description.
The reflection factor r can then be expressed as the quotient of voltage UR, proportional to the field strength of the returning wave, and voltage UH, proportional to the field strength of the departing wave:                     r        =                              U            R                                U            H                                              (                  equation          ⁢                      xe2x80x83                    ⁢          9                )            
It is dependent in the following way on the wave resistances
Z1, Z2 of the two mediums at a border area:                     r        =                                            Z              2                        -                          Z              1                                                          Z              2                        +                          Z              1                                                          (                  equation          ⁢                      xe2x80x83                    ⁢          10                )            
Thus the following can be equated:                                           U            R                                U            H                          =                                            Z              2                        -                          Z              1                                                          Z              2                        +                          Z              1                                                          (                  equation          ⁢                      xe2x80x83                    ⁢          11                )            
Which is transformed to:                               Z          2                =                                                            U                H                            +                              U                R                                                                    U                H                            -                              U                R                                              ·                      Z            1                                              (                  equation          ⁢                      xe2x80x83                    ⁢          12                )            
In relation to the gas/filling material border area this equation is:                               Z                      filling            ⁢                          xe2x80x83                        ⁢            material                          =                                                            U                H                            +                              U                R                                                                    U                H                            -                              U                R                                              ·                      Z            Gas                                              (                  equation          ⁢                      xe2x80x83                    ⁢          13                )            
By comparing the two equations 8 and 13 the following relationship is obtained:                               1                                    ϵ              r                                      =                                            U              H                        +                          U              R                                                          U              H                        -                          U              R                                                          (                  equation          ⁢                      xe2x80x83                    ⁢          14                )            
or by transforming equation 14:                               ϵ          r                =                              (                                                            U                  H                                -                                  U                  R                                                                              U                  H                                +                                  U                  R                                                      )                    2                                    (                  equation          ⁢                      xe2x80x83                    ⁢          15                )            
Thus the sought-after DK value sr can be determined by calculating the amplitudes of the departing and returning wave at the reflection point in accordance with equation 15 if, as assumed, the wave resistance of the conductor is inversely proportional to the square root of the dielectric constants xcex5r of the wave-conducting medium.
For conductors for which this proportionality does not apply the indicated solution can be applied in the same way if the relationship between the change in the dielectric constants of the medium in which the wave is propagated and the change in the corresponding wave resistance is known.
To determine the voltages of the departing and returning wave at the point of reflection the following method is preferred: the voltage of the departing wave can be determined from a measurement of the echo amplitude of a precisely defined, known reflection point. This reflection point may lie within the sensor, e.g., in a known conductor impedance modification in the line between the electronic unit and the probe. With equal success it may be a part of the measuring section within the container, e.g., the total reflection of the wave at the metallic container floor in the case of an empty container or the open-circuited or short-circuited conductor end of the probe in the case of an empty container. When there is a constant transmission voltage in the sensor it is sufficient to measure this amplitude of the departing wave only once (if necessary with factory adjustment of the device) and to permanently store the value. With a knowledge of the sectional attenuations in the wave propagation the amplitude of the departing wave can thus be calculated. The amplitude of the returning wave at the reflection point can be obtained by determining the echo amplitude of the reflection point from the momentarily received echo profile.